Embodiments of the present invention relate generally to subsea equipment involved in the transport of hydrocarbon production fluids from a production site at the sea floor to a sea surface or land based host facility. More specifically, embodiments of the present invention are related to a system that is designed for management of barrier and lubrication fluid pressures in a subsea motor and pump module.
A process fluid in subsea hydrocarbon production is typically a multiphase fluid comprising oil and gas and eventually solid matter, which is extracted from an underground reservoir. A motor/pump module is arranged on the sea floor and configured for transport of the process fluid from the reservoir to a surface or land based host facility. The motor/pump module is frequently subjected to substantial variations in pressure in the pumped medium, as well as substantial transitional loads during pump start and stop sequences, for example, the medium pressure at the suction side of the pump may be in the order of hundreds of bar, requiring corresponding measures in the motor/pump module to prevent process fluid and particulate matter from immigration from the pump interior into a motor housing, and into bearings and seals of the motor/pump module.
For the purpose of pumping a multiphase fluid in subsea production, screw rotor pumps are used. The screw rotor pump is a positive displacement type of pump having two screw shafts that are driven in rotation with intermeshing gears, between which a specific volume of fluid is displaced in the axial direction of the screws from a suction side of the pump to be discharged on the pressure side of the pump. The screws are journalled in bearings in a pump housing, and are drive-connected to a motor arranged in a motor housing. In case of a twin rotor screw pump, intermeshing timing gears carried on the screw shafts provide synchronization of the rotary motion. The motor housing interior is hydraulically separated from the pump housing interior by a seal arrangement, where the drive shaft is journalled to extend for connection with the pump rotor shaft. The pump bearings are separated from the pump medium by seal arrangements at both ends of the pump.
A hydraulic fluid in the motor housing is controlled at a pressure above the internal pressure of the pump, acting as a barrier which prevents intrusion of process fluid and particles into the motor housing via the seal and bearing arrangement. As a result of the pressure difference, a leak flow of hydraulic fluid along the drive shaft is unavoidable. The leakage rate is dependent on fluid properties, differential pressure, the transient operating conditions of the pump, and the tightness of the seal(s). The leakage is compensated by refilling the motor housing from an external supply of hydraulic fluid. Likewise, hydraulic fluid is used for lubrication of pump bearings and timing gears. The pressure in the pump lubrication fluid is to be maintained above the pressure of the pumped medium internally of the pump, in order to prevent intrusion of process fluid and particles into pump bearings, seals and timing gears. Leakage via the pump seals into the pumped medium is compensated by refilling from an external supply of hydraulic fluid.
The motor and pump can be drive-connected inside the motor housing, or outside the motor housing. For instance, the motor and pump can share one and the same shaft with no separate coupling that connects them in a driving relation. In other designs the pump shaft can be coupled to the motor shaft inside the motor housing. In still other designs, the motor and pump is drive-connected by means of a coupling located in a coupling chamber defined between the motor housing and the pump. However, in all alternatives it is desirable to maintain at all times a pressure difference over the interfaces, i.e. between the motor housing, the coupling chamber when present, and the pump lubrication system and the pumped medium, respectively.
Conventionally, a motor barrier fluid and a pump lubrication fluid are each supplied from a host facility, and leakage compensation as well as pressure control is managed from the host facility, usually via an umbilical. As subsea hydrocarbon production sites are increasingly installed and operated at increasing depths and step-out distances, the response times and control requirements in lubrication and cooling systems increase correspondingly. As a consequence, there is a rising need for a barrier fluid and lubrication system that operates with improved control requirements and which provides increased reliability in operation.